Free Astronomy Magazine January-February 2015

SPACE CHRONICLES For each of the stars the team used the 1.8-metre Auxiliary Telescopes to feed light to the VLTI. Where strong exozo- diacal light was pre- sent they were able to fully resolve the extended discs of dust, and separate their faint glow from the dominant light of the star. As a by-prod- uct, these observa- tions have also led to the discovery of new, unexpected stellar companions orbiting around some of the most massive stars in the sample. "These new companions sug- gest that we should revise our current un- derstanding of how many of this type of star are actually dou- ble," says Lindsay Marion, lead author of an additional pa- per dedicated to this complementary work using the same data. By analysing the prop- erties of the stars surrounded by a disc of exozodiacal dust, the team found that most of the dust was de- tected around older stars. This result was very surprising and raises some questions for our understanding of planetary systems. Any known dust production caused by collisions of planetesimals should diminish over time, as the number of planetesi- mals is reduced as they are destroy- ed. The sample of observed objects also included 14 stars for which the detection of exoplanets has been reported. All of these planets are in the same region of the system as the dust in the systems showing T his image beautifully captures the zodiacal light, a triangular glow seen best in night skies free of overpowering moonlight and light pollution. The photograph was taken at ESO’s La Silla Obser- vatory in Chile in September 2009, facing west some minutes after the Sun had set. A sea of clouds has settled in the valley below La Silla, which sits at an altitude of 2400 metres, with lesser peaks and ridges poking through the mist. The zodiacal light is sunlight reflected by dust particles between the Sun and Earth, and is best seen close to sunrise or sunset. As its name implies, this celestial glow appears in the ring of constellations known as the zodiac. These are found along the ecliptic, which is the eastward apparent “path” that the Sun traces across Earth’s sky. [ESO/Y. Beletsky] exozodiacal light. The presence of exozodiacal light in systems with planets may create a problem for further astronomical studies of exo- planets. Exozodiacal dust emission, even at low levels, makes it significantly harder to detect Earth-like planets with direct imaging. The exozodia- cal light detected in this survey is a factor of 1000 times brighter than the zodiacal light seen around the Sun. The number of stars containing zodiacal light at the level of the Solar System is most likely much higher than the numbers found in the survey. These observations are therefore only a first step towards more detailed studies of exozodia- cal light. “The high detection rate found at this bright level suggests that there must be a significant number of sys- tems containing fainter dust, un- detectable in our survey, but still much brighter than the Solar Sys- tem’s zodiacal dust,” explains Oliv- ier Absil, co-author of the paper, from the University of Liège. “The presence of such dust in so many sys- tems could therefore become an obstacle for future observations, which aim to make direct images of Earth-like exoplanets.” n